Field of the Invention
The present invention relates to methods and apparatus for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). The spot defines effectively the field of view (FOV) of the scatterometer, while underfilling the grating simplifies mathematical reconstruction of the target as it can effectively be regarded as infinite. The cost of “real estate” occupied by such large targets is a major problem, however. It would be useful to include more metrology targets across a substrate. It would be particularly interesting to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, for example 5 μm by 5 μm, so they can be positioned in amongst product features, rather than in the scribe lane. Metrology for overlay has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Detection of the diffraction orders in the form of dark field images enables overlay measurements to be made on smaller targets. Examples of dark field metrology can be found in international patent applications WO 2009/078708 and WO 2009/106279.
While detection using dark field imaging allows useful measurements of overlay to be made on small, overfilled targets, it does not provide suitable signals for other types of metrology. Moreover, the overlay measurements depend to a large extent on the assumption that all parameters except overlay are unchanged. Reconstruction of the target structure to determine parameters such as critical dimension (CD), side wall angle (SWA) or height, still requires large targets, which are underfilled by the illumination spot (field of view). Patent application US 2012/0123748A1 describes such a reconstruction process in more detail. In U.S. Pat. No. 6,850,333, it is proposed to form an illumination spot that is elongated. This spot is used to illuminate as many lines as possible in the direction of periodicity of the grating, without overfilling it in the direction of the lines (the direction transverse to periodicity). Nevertheless the requirement for underfilling the grating limits the extent to which it can be shrunk. To reduce the size of the illumination spot so as to allow the grating to be smaller brings a reduction in the amount of radiation that can be captured in a given time, leading to slowness in the measurement process, and/or a loss of accuracy.